Hot melt pressure sensitive adhesives usually include a mixture of a polymer or copolymer (in the following, the term "polymer" will be used for both homopolymers and copolymers), preferably a block copolymer, a petroleum tackifier resin and optionally further components such as an extender oil (also named processing oil), fillers and some additives like antioxidants or colorants.
The above polymer often is an S-I-S (styrene-isoprene-styrene) block copolymer having polystyrene and polyisoprene block segments. In such a case, the adhesive composition may additionally include some polystyrene-polyisoprene diblocks.
The petroleum-based resin exerts an important influence on the physico-chemical properties of the adhesive such as its hot melt viscosity, compatibility to aromatic solvents, shear properties and tack properties. To attain the optimum profile of a low melt viscosity, good compatibility to organic solvents, high tack and shear properties without losing high temperature shear performance, conventional petroleum-based tackfying resins used to contain a certain amount of aromatic modifications. Prior art resins with no aromatic modification failed to attain the above properties to a satisfactory extent.
The following patents and/or patent applications may be mentioned as related documents of interest:
U.S. Pat. No. 4,411,954 describes the use of aliphatic resins prepared as described in U.S. Pat. No. 3,577,398 in S-I-S based hot melt pressure sensitive adhesives.
EP 0447855 describes the use of aliphatic and aromatic modified aliphatic resin in S-I-S based hot melt pressure sensitive adhesives. The used levels of aromatic modification were between 11 and 15%, and styrene was mentioned as the aromatic monomer.
U.S. Pat. No. 4,623,698 describes the use of aliphatic and aromatic modified aliphatic resins with softening points from 0 to 80.degree. C. as tackifier in styrene butadiene polymers.
U.S. Pat. No. 4,078,132 describes the preparation of aliphatic and aromatic modified aliphatic resins from heat-soaked piperylene streams. Isobutylene and isoamylene are described to act as chain transfer agent and alpha-methylstyrene (AMS) is described to be used as a aromatic modifier. Some applications are described using resins with softening points between 70 and 85.degree. C.
Wo 95/16755 describes a hot melt adhesive formulation using a S-I-S polymer, an aromatic modified aliphatic resin and an extender oil. The aromatic modification caused hot melt pressure sensitive adhesive formulations having a low viscosity and enhanced tack properties.
Aromatic modified tackifying resins, however, are disadvantageous for a number of reasons. Firstly, aromates are relatively expensive, and the use of no aromate would make the resin cheaper. Secondly, aromatic modified resins are less suitable for hydrogenation, hydrotreatment and quenching processes to improve the resin color, before they are used in any application. Thirdly, aromatic modified aliphatic resins result in lower shear performance at higher temperatures in hot melt based pressure sensitive applications. It would therefore be desirable to provide a cheaper petroleum-based resin which exhibits similar or even better application properties than aromatic modified resins.
It is also desirable to provide aliphatic resins which are useful in hot melt pressure sensitive adhesive compositions and combine a low melt viscosity with excellent tack and shear properties, and a hot melt pressure sensitive adhesive containing such resins.
Accordingly, it was found that aliphatic resins having an increased amount of cyclic structures had the desired R&B softening points and molecular weights and resulted in a low viscosity hot melt pressure sensitive adhesive composition having excellent tack and shear properties without losing its high temperature shear performance. The resins of the present invention are particularly distinguished from aliphatic resins of the state of the art in that they contain a higher amount of cyclic structures. The term "cyclic structure" is defined as any structure feature which serves to increase the level of crosslinking or network structure in the resin. These types of structural features can be the result of, but are not limited by, the following reactions: back biting of its growing polymer chain to incorporate cyclic structures along a backbone and/or at chain termination, dimerization of monomers to produce reactive cyclic structures which can be incorporated in the polymer chains, intramolecular linking of pendent groups within a polymer chain, and crosslinking between short polymer chains to form highly networked structures. This can be confirmed by determining a number of physicochemical properties which are indicative for cyclic structures. It was further found that the amount of cyclic structures allows an independent control of the R&B softening point and the molecular weight of a resin.